Add new test demonstrating endless loop with incorrect G71 parameters

[petterreinholdtsen]

I found this issue in issue #1146. This example g-code program causes
the AXIS LinuxCNC GUI to get completely stuck while its memory use explode.
I decided to write a small test case to demonstrate the problem.
Discovered that the program cause the rs274 program to generate an
endless stream of repeated instructions, which I suspect is the cause of
the memory explosion.

[petterreinholdtsen]

For the record, this is a gdb backtrace from the endless look, after breaking in fprintf:

#0  0x00005555555be851 in fprintf (__fmt=<optimized out>, 
    __stream=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/stdio2.h:100
#1  STRAIGHT_FEED (line_number=-1, x=3.064721359549996, y=0, 
    z=-14.039999999999999, a=0, b=0, c=0, u=0, v=0, w=0)
    at emc/sai/saicanon.cc:379
#2  0x00007ffff7e8725c in Interp::convert_straight (this=0x7ffff7f7f010, 
    move=move@entry=10, block=0x7ffff7f801b0, settings=0x7ffff7f80020)
    at emc/rs274ngc/interp_convert.cc:4781
#3  0x00007ffff7eb4f55 in motion_machine::straight_move (this=<optimized out>, 
    end=...) at emc/rs274ngc/interp_g7x.cc:952
#4  0x00007ffff7eb1e1f in straight_segment::dive (this=<optimized out>, 
    location=..., x=-3.064721359549996, output=0x55555571c7b0, 
    fast=<optimized out>) at emc/rs274ngc/interp_g7x.cc:212
#5  0x00007ffff7eb1fb2 in g7x::pocket (this=0x7fffffffd460, cycle=3, 
    location=..., p=..., out=0x55555571c7b0) at emc/rs274ngc/interp_g7x.cc:767
#6  0x00007ffff7eb73f8 in g7x::do_g71 (this=this@entry=0x7fffffffd460, 
    out=<optimized out>, cycle=cycle@entry=2, x=x@entry=15, z=z@entry=0, u=0, 
    w=<optimized out>, d=0.040000000000000001, i=0.5, r=1, 
    do_rotate=<optimized out>) at emc/rs274ngc/interp_g7x.cc:84
#7  0x00007ffff7eb3e5f in g7x::do_g72 (r=<optimized out>, i=<optimized out>, 
    d=<optimized out>, w=<optimized out>, u=<optimized out>, z=<optimized out>, 
    x=<optimized out>, cycle=<optimized out>, out=<optimized out>, 
    this=<optimized out>) at emc/rs274ngc/interp_g7x.cc:745
#8  Interp::convert_g7x (this=this@entry=0x7ffff7f7f010, mode=mode@entry=712, 
    block=block@entry=0x7ffff7f801b0, settings=settings@entry=0x7ffff7f80020)
    at emc/rs274ngc/interp_g7x.cc:1195
#9  0x00007ffff7e8aa60 in Interp::convert_motion (
    this=this@entry=0x7ffff7f7f010, motion=<optimized out>, 
    block=block@entry=0x7ffff7f801b0, settings=settings@entry=0x7ffff7f80020)
    at emc/rs274ngc/interp_convert.cc:3757
#10 0x00007ffff7e8b0dc in Interp::convert_g (this=this@entry=0x7ffff7f7f010, 
    block=block@entry=0x7ffff7f801b0, settings=settings@entry=0x7ffff7f80020)
    at emc/rs274ngc/interp_convert.cc:2358
#11 0x00007ffff7e9c56c in Interp::execute_block (
    this=this@entry=0x7ffff7f7f010, block=block@entry=0x7ffff7f801b0, 
    settings=settings@entry=0x7ffff7f80020)
    at emc/rs274ngc/interp_execute.cc:304
#12 0x00007ffff7f10aaf in Interp::_execute (this=this@entry=0x7ffff7f7f010, 
    command=<optimized out>) at emc/rs274ngc/rs274ngc_pre.cc:504
#13 0x00007ffff7f10e8f in Interp::execute (this=0x7ffff7f7f010, 
    command=<optimized out>) at emc/rs274ngc/rs274ngc_pre.cc:533
#14 0x00005555555c171b in interpret_from_file (do_next=2, 
    block_delete=block_delete@entry=0, print_stack=0) at emc/sai/driver.cc:287
#15 0x00005555555bcb09 in main (argc=<optimized out>, argv=0x7fffffffdf60)